1. Field of the Invention
The present invention relates to a magneto-resistive effect device adapted to read the magnetic field intensity of magnetic recording media or the like as signals, a thin-film magnetic head comprising that magneto-resistive effect device, and a head gimbal assembly and a magnetic disk system including that thin-film magnetic head.
2. Explanation of the Prior Art
In recent years, with an increase in the recording density of hard disks (HDDs), there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (hereinafter often called the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together.
The magneto-resistive effect device (CIP-GMR device) of the so-called CIP (current in plane) structure that operates on currents flowing parallel with the film plane of the device—called a spin valve GMR device—is now widely employed as the reproducing head. The spin valve GMR device of such structure is positioned between upper and lower shield layers each formed of a soft magnetic metal film, and sandwiched between insulating layers called gap layers. Recording density in the bit direction is determined by the gap (reproducing gap) between the upper and lower shield layers.
With an increase in the recording density, there has been a growing demand for narrower shield gaps and narrower tracks. As the reproducing head track grows narrow with a decreasing device height, so does the device area; however, with the prior art structure, there is an operating current limited from the standpoint of reliability, because there is heat dissipation efficiency decreasing with a decreasing area.
To solve such a problem, there is a GMR device of the CPP (current perpendicular to plane) structure (CPP-GMR device) proposed in the art, in which upper and lower shield layers and a magneto-resistive effect device are connected electrically in series to make do without any insulating layer between the shields. This technology is thought of as inevitable to achieve such recording densities as exceeding 200 Gbits/in2.
Such a CPP-GMR device has a multilayer structure comprising a first ferromagnetic layer and a second ferromagnetic layer between which an electroconductive, nonmagnetic intermediate layer is sandwiched. A typical multilayer structure for the spin valve type CPP-GMR device comprises, in order from a substrate side, a lower electrode/antiferromagnetic layer/first ferromagnetic layer/electroconductive, nonmagnetic layer/second ferro-magnetic layer/upper electrode stacked together in order.
The direction of magnetization of the first ferro-magnetic layer that is one of the ferromagnetic layers remains fixed such that when the externally applied magnetic field is zero, it is perpendicular to the direction of magnetization of the second ferromagnetic layer. The fixation of the direction of magnetization of the first ferromagnetic layer is achieved by the exchange coupling of it with an antiferromagnetic layer provided adjacent to it, whereby unidirectional anisotropic energy (also called the “exchange bias” or “coupled magnetic field”) is applied to the first ferromagnetic layer. For this reason, the first ferromagnetic layer is also called the fixed magnetization layer. By contrast, the second ferromagnetic layer is also called the free layer. Further, if the fixed magnetization layer (the first ferromagnetic layer) is configured as a triple-layer structure of a ferromagnetic layer/nonmagnetic metal layer/ferromagnetic layer (the so-called “multilayer ferri-structure” or “synthetic pinned layer”), it is then possible to effectively increase the exchange coupling force from the antiferromagnetic layer, and to reduce the influences on the free layer of a static magnetic field resulting from the fixed magnetization layer. Thus, the “synthetic pinned structure” is now in extensive use.
However, a further slimming-down of the magneto-resistive effect device is in great need so as to meet a recent demand for ultra-high recording density. In such situations, there is a novel GMR device structure put forward, which has a basic structure comprising a simple triple-layer arrangement of ferromagnetic layer/non-magnetic intermediate layer/ferromagnetic layer, as set forth in U.S. Pat. No. 7,019,371B2 or U.S. Pat. No. 7,035,062B1. In such a new GMR device structure, the two ferromagnetic layers are exchange coupled together such that their magnetizations are anti-parallel with each other. And, according the proposal of U.S. Pat. No. 7,035,062B1, a permanent magnet 500 is located in the depth position facing away from the ABS corresponding to the medium opposite plane of the device, as shown in FIG. 14, so that under the action of a bias magnetic field given out of the permanent magnet 500, there is an initial state created in which the magnetizations of two magnetic layers 411 and 415 are inclined about 45° with respect to the track width direction. Upon detection of a signal magnetic field from the medium in the initial state of the device, the directions of magnetization of the two magnetic layers change as if scissors cut paper, with the result that there is a change in the resistance value of the device. In the present disclosure, this new device structure may be called the “scissors type GMR device”.
As such a “scissors type GMR device” is applied to the CPP-GMR device, it enables the read gap G that is the gap between the upper and lower shield layers 501 and 505 to be much narrower than could be achieved with a general spin valve type CPP-GMR device (see FIG. 14). Specifically, it is possible to make do without the anti-ferromagnetic layer thought of as necessary for the general spin valve type CPP-GMR device, and dispense with the ferromagnetic layers in the “synthetic pinned structure” as well.
With a head structure using the “scissors type GMR device”, however, it is necessary to make the permanent magnet 500 thicker to allow the permanent magnet 500 of CoPt or the like located in the rear of the device as shown in FIG. 14 to give out a bias magnetic field intensity high enough to create the initial state. That the permanent magnet 500 grows thick means that it is impossible to take full advantage of the merit of the scissors type GMR device of being narrow in the read gap G. Further, as shown in FIG. 14, the narrower the read gap G, the smaller the gap between the permanent magnet 500 and the upper and lower shield layers 501, 505 becomes: there is a problem that the bias magnetic field given out of the permanent magnet 500 goes through the upper and lower shield layers 501, 505, and becomes insufficient to make sure a sufficient resistance change in the device.
The situations being like this, an object of the present invention is to provide a magneto-resistive effect device of improved reliability that enables a structure capable of having a narrowed read gap (the gap between the upper shield and the lower shield) to be adopted to meet the recently demanded ultra-high recording density, allows a stable bias magnetic field to be applied in simple structure, and obtain a stable magneto-resistive effect change as well as a thin-film magnetic head comprising that magneto-resistive effect device, and a head gimbal assembly and magnetic disk system including that thin-film magnetic head.